I have a variation on this question.
No problem when I backed up my music library and Roon database to a single USB drive. Liked the idea of having multiple backups on different USB drives. After completing "rsync -avrz /music /mnt" I then issue the "cd /" and "umount /mnt" commands before taking the backup drive #1 off the Splayer and then plugging in backup drive #2.
More often than not, I get a "Target is busy" statement from Terminal, and My only work around at this point has been to shut down my Splayer, disconnect the USB drive, wait a few minutes and then restart the Splayer.
I think I must be doing something wrong, but no idea what...
Here's the abbreviated log file after a successful backup to Drive #1
sent 1,659,228,153 bytes received 187,999 bytes 11,895,456.29 bytes/sec
total size is 773,107,058,639 speedup is 465.89
root@streamplayer / $ cd /
root@streamplayer / $ umount /mnt
AT THIS POINT I REMOVED DRIVE #1 & SWAPPED TO DRIVE #2 USING THE SAME CABLE PLUGGED INTO THE SAME SPLAYER USB CONNECTION
root@streamplayer / $ blkid
/dev/sda1: UUID="b1a051b9-66ea-42e5-8af6-1d62221a5bb3" TYPE="ext4" PARTUUID="000bf16b-01"
/dev/sdb1: LABEL="Seagate Expansion Drive" UUID="20A691E6A691BCAA" TYPE="ntfs" PTTYPE="atari" PARTUUID="d7a3afb5-01"
root@streamplayer / $ umount /dev/sdb1
umount: /media/Seagate Expansion Dr: target is busy.
Thoughts? Recommendations? Thanks in advance!!!